1. Field of the Invention
This invention relates to optical amplifiers and in particular optical amplifiers involving doped materials.
2. Art Background
Considerable recent research has involved the development of optical amplifiers useful in optical communications. Typically, these amplifiers involve a waveguide formed in a glassy material (a material that has no long-range ordering and is characterized by an absence of Bragg peaks in X-ray diffraction and/or a glass transition observed in differential scanning calorimetry) with a rare earth dopant present in the waveguide core and with a region of lower refractive index surrounding the core. Since the glassy material host does not substantially affect the emission spectrum of the dopant, the rare earth material is chosen to have a spectral emission line corresponding to a wavelength at which optical communication is to be performed. For example, most long-haul optical communication is performed either at 1.3 .mu.m or 1.55 .mu.m.
Optical amplifiers at 1.55 .mu.m have been demonstrated. These amplifiers involve a waveguide fiber having erbium, that emits at 1.53 to 1.58 .mu.m, present in the core at concentrations typically in the range 10 to 1000 parts per million. In operation optical power of wavelength 0.975 or 1.48 .mu.m is introduced into the waveguide core along with a signal at wavelength 1.55 .mu.m. The optical power induces a transition in the erbium that populates a state capable of decaying to emit at 1.55 .mu.m, and the signal induces this decay from the populated state. Thus, the output from the amplifier involves a signal at 1.55 .mu.m that has an intensity approaching that of the combined power and signal inputs. In this manner, an optical signal is amplified without conversion to an electrical signal, amplification of this signal electronically and then conversion back to an optical signal.
Although the erbium/glass optical fiber configuration shows greater promise at 1.55 .mu.m, amplification at 1.3 .mu.m for this system is not possible. Alternatively, approaches have been proposed for the 1.3 .mu.m wavelength. For example, investigations have involved the use of neodymium in a silica glass fiber waveguide. Although the amplified output of a signal at 1.32 to 1.37 .mu.m is possible, this output is accompanied by a corresponding signal at 106 .mu.m that is three times more intense. The presence of this more intense parasitic signal together with a strong absorption in the silica fiber at 1.37 .mu.m, and the presence of excited state absorption makes this approach less than entirely desirable. Although a variety of dopants have been utilized in glassy materials to form amplifiers in a waveguide configuration, a promising approach at 1.3 .mu.m for such configurations is not presently available.